Current caustic solution regeneration systems often use air as a supply of oxygen with oxygen and water being used to convert mercaptides to disulfide oil and regenerated caustic. Referring to prior art FIG. 1, air B1 from air source 20 (typically just the atmosphere around the plant in which regeneration is being done) is compressed by compressor 22 and regulated by valve 23, and then fed as input stream B into oxidizer tower 40 which is typically a co-current flow tower (A plant may utilize a second source of air 21 to provide air B2 to replace or supplement air B1). Spent caustic A1 from caustic source 10 is also fed via heat exchanger 15 as input stream A into oxidizer tower 40, generally with catalyst A2 from catalyst source 11 mixed in while caustic A1 passes through mixing device 13. In oxidizer tower 40, the air B and caustic A are contacted for oxidation, and regenerated caustic C is transported via pump 24 for reuse in the treating process unit 25. The regeneration reaction only uses the oxygen portion of the air supplied to the tower. As such, the unused gasses contained in the air are a waste product of the reaction and are vented from the tower as a stream D of mixed gas and hydrocarbons, with oxygen and nitrogen being large components of the vented stream D. In some instances, natural gas from gas source 30 is also fed into oxidizer tower 40 or into downstream piping in order to minimize the risk of explosion posed by the vented stream D.
After being vented from oxidizer tower 40, the vent stream D of gas and hydrocarbons will often be scrubbed in a caustic scrubbing system to minimize entrained oils prior to disposal. In FIG. 1, the stream is fed into off gas absorber (“OGA”) 50. In OGA 50, the vented waste D from tower 40 is contacted with fresh caustic I from source 12. From OGA 50, caustic A3 is pumped by pump 14 back into input stream A, and the “scrubbed” nitrogen, oxygen, and other gasses E are fed into KO drum 60 prior to disposal to the atmosphere 90 via a downstream combustion device 70 Liquids collected in the KO drum 60, are returned to the caustic system.
The air for the oxidizer is generally supplied by an air compressor 22 or from a plant air system. Designs of this type use greater than the stochiometric oxygen requirements thereby increasing air supply and purge requirements.
Depending on the concentrations of the spent caustic material, variations to FIG. 1 include methods to cool the recycle stream A3 via heat exchange with the feed stream A1 and cooling water to moderate the overall temperature in the oxidizer 40 as the caustic regeneration is an exothermic reaction.